The present invention relates a method of and device for communications that use a multi-carrier modem system, and, more particularly, concerns a communication device which realizes data communication through the existing communication lines by using a system such as the DMT (Discrete Multi Tone) modem system and the OFDM (Orthogonal Frequency Division Multiplex) modem system and a communication method for such a communication device. However, the present invention is not intended to be limited to the communication device for carrying out data communication through the DMT modem system, and is applicable to any communication device for carrying out cable communication and radio communication through normal communication lines by using the multi-carrier modem system and a single carrier modem system.
The conventional communication methods will be explained here. For example, in the wide band CDMA (W-CDMA: Code Division Multiple Access) using the SS (Spread Spectrum) system, turbo codes have been proposed as error-correction codes that greatly exceed convolutional codes in their performances. In the turbo code, a list formed by interleaving an information list is encoded in parallel with a known coding list, and the turbo code is one of the error-correction codes that have attracted the greatest public attention at present, and is said to provide characteristics close to Shannon limit. In the above-mentioned W-CDMA, since the performances of the error-correction code give great effects on the transmission characteristics in the voice transmission and data transmission, the application of the turbo code makes it possible to greatly improve the transmission characteristics.
Operation of transmitting and receiving systems of a conventional communication device using the turbo code will be explained in detail. FIG. 6 is a drawing that shows the construction of a turbo encoder used in the transmitting system. In FIG. 6(a), reference number 101 is a first recursive system convolutional encoder that subjects an information list to a convolutional encoding process to output redundant bits, 102 is an interleaver, and 103 is a second recursive system convolutional encoder that subjects the information list that has been switched by the interleaver 102 to a convolutional encoding process to output redundant bits. FIG. 6(b) is a drawing that shows the inner structures of the first recursive system convolutional encoder 101 and the second recursive system convolutional encoder 103, and the two recursive system convolutional encoders are encoders that only output redundant bits respectively. Moreover, the interleaver 102, which is used in the turbo encoder, randomly switches information bit lists.
The turbo encoder, which is arranged as described above, simultaneously outputs an information bit list: x1, a redundant bit list: x2 obtained by encoding the information bit list through the operation of the first recursive system convolutional encoder 101, and a redundant bit list: x3 obtained by encoding the information bit list that has been interleaved through the operation of the second recursive system convolutional encoder 103.
FIG. 7 is a drawing that shows the construction of the turbo decoder that is used in the receiving system. In FIG. 7, reference number 111 indicates a first decoder that calculates a logarithm likelihood ratio from the received signals y1 and y2. Reference numbers 112 and 116 indicate adders, 113 and 114 indicate interleavers, 115 indicates a second decoder that calculates a logarithm likelihood ratio from the received signals y1 and y3. Reference number 117 indicates a de-interleaver, 118 indicates a judging device which judges the output of the second decoder 115 to output an estimated value of the original information bit list. The received signals y1, y2, y3 are signals that are formed by allowing the information bit list x1 and the redundant bit lists x2, x3 to include influences from noise and phasing in the transmission path.
In this turbo decoder, first, the first decoder 111 calculates the logarithm likelihood ratio: L (x1kxe2x80x2) (where k refers to the time) of estimated information bit: x1kxe2x80x2 from received signals: y1k and y2k. In this case, the logarithm likelihood ratio: L (x1kxe2x80x2) is represented by the following equation:                                                                         L                ⁡                                  (                                      x                                          1                      ⁢                      k                                        xe2x80x2                                    )                                            =                              xe2x80x83                            ⁢                                                y                                      1                    ⁢                    k                                                  +                                  La                  ⁡                                      (                                          x                                              1                        ⁢                        k                                                              )                                                  +                                  Le                  ⁡                                      (                                          x                                              1                        ⁢                        k                                                              )                                                                                                                          =                              xe2x80x83                            ⁢                              Ln                ⁢                                  xe2x80x83                                ⁢                                                      Pr                    (                                                                                            x                                                      1                            ⁢                            k                                                                          =                                                  1                          ⁢                                                      "LeftBracketingBar"                                                          {                              Y                              }                                                                                                                          )                                                                            Pr                    (                                                                                            x                                                      1                            ⁢                            k                                                                          =                                                  0                          ⁢                                                      "LeftBracketingBar"                                                          {                              Y                              }                                                                                                                          )                                                                                                                              (        1        )            
In equation (1), Le (x1k) represents external information, La (x1k) represents preliminary information that is external information preceding by one, Pr (x1k=1|{Y}) represents the probability of an actually transmitted information bit: x1k being 1 under the condition that the entire list {Y} of the received signals has been received, and Pr (x1k=0{y}) represents the probability of an actually transmitted information bit: x1k being 0 under the condition that the entire list {Y} of the received signals has been received. In other words, equation (1) finds a ratio of the probability of the information bit: x1k becoming 1 to the probability of the information bit: x1k being 0.
The adder 112 calculates external information to be given to the second decoder 115 from a logarithm likelihood ratio that is the result of the above-mentioned calculation. Based upon the equation (1), the external information: Le (x1k) is represented by the following equation:
Le (x1k)=L(x1k)xe2x88x92y1kxe2x88x92La (x1k)xe2x80x83xe2x80x83(2)
Since no preliminary information has been given at the time of the first decoding process, La (x1k)=0.
In the interleavers 113 and 114, in order to make the received signal: y1k and the external information: Le (x1k) coincident with the time of the received signal: y3, the signals are re-arranged. Then, in the same manner as the first decoder 111, based upon the received signal: y1 and the received signal: y3 as well as the external information: Le (x1k) preliminarily calculated, the second decoder 115 calculates a logarithm likelihood ratio: L (x1kxe2x80x2). Thereafter, in the same manner as the adder 112, the adder 116 calculates the external information Le (x1k) by using equation (2). At this time, the external information, rearranged by the interleave 117, is fed back to the first decoder 111 as the preliminary information: La (x1k).
Finally, in the turbo decoder, the above-mentioned processes are repeatedly executed predetermined times so that it is possible to calculate a logarithm likelihood ratio with higher precision, and the judgment device 118 makes a judgment based upon this logarithm likelihood ratio, thereby estimating the bit list of the original information. More specifically, for example, the logarithm likelihood ratio shows that xe2x80x9cL (x1kxe2x80x2) greater than 0xe2x80x9d, the estimated information bit: x1kxe2x80x2 is judged as 1, while it shows that xe2x80x9cL (x1kxe2x80x2)xe2x89xa60xe2x80x9d, the estimated information bit: x1kxe2x80x2 is judged as 0.
In this manner, in the conventional communication method, by using the turbo code as the error-correction code, even in the case when the signal point-to-point distance becomes closer as the modulation system is multi-valued, it becomes possible to greatly improve the transmitting property in the voice transmission and data transmission, and consequently to obtain characteristics superior to the known convolutional codes.
However, in the conventional communication method, in order to carry out an error correction with high precision, the turbo encoding process is carried out on all the information lists on the transmitting side, and on the receiving side, all the encoded signals are decoded, and a soft-judgment is then executed thereon. More specifically, for example, in the case of 16 QAM, a judgment is made with respect to all the 4-bit data (0000 to 1111: 4-bit constellation), and in the case of 256 QAM, a judgment is made with respect to all the 8-bit data. Therefore, conventionally, the application of the conventional communication method that carries out judgments on all the data as described above causes a problem of an increase in the number of calculations in the encoder and decoder in response to the multi-valued levels.
Moreover, the demodulation process is executed by carrying out repeated calculations with or without an influence from noise, that is, irrespective of the state of the transmission path. Therefore, even in the case of a good state of the transmission path, the same number of calculations and the same amount of delay as in the case of a bad state thereof are required.
The present invention has been devised to solve the above-mentioned problems, and its object is to provide a method of and device for communications for such a device, which is applicable to any communication system using the multi-carrier modem system and the single-carrier modem system, and makes it possible to achieve a reduction in the number of calculations and to provide a good transmitting property, even in the case when there is an increase in the constellation due to multi-valued levels, and also to greatly reduce the number of calculations and the calculation processing time in the case of a good transmission path.
A communication device in accordance with the present invention, which uses turbo codes as error-correction codes, is provided with: a turbo encoder (corresponding to a turbo encoder 1 in an embodiment which will be described later) which carries out a turbo encoding process on lower two bits in transmission data to output an information bit list of the two bits, a first redundant bit list generated in a first convolutional encoder having the information bit list of the two bits as an input and a second redundant bit list generated in a second convolutional encoder to which the respective information bit lists that have been subjected to interleave processes are switched and input; a first decoder unit (corresponding to a first decoder 11, an adder 12 and interleavers 13, 14) which extracts the information bit list of the two bits and the first redundant bit list from a received signal, and calculates the probability information of estimated information bits by using the results of the extraction and probability information that has been given as preliminary information (in some cases, not given); a second decoder unit (corresponding to a second decoder 15, an adder 16 and a di-interleaver 17) which extracts the information bit list of the two bits and the second redundant bit list, and again calculates the probability information of estimated information bits by using the results of the extraction and the probability information from the first decoder unit to inform the first decoder unit of the results as the preliminary information; a first estimating unit (corresponding to a first judging device 18 and a second judging device 20) which, based upon the results of the calculation processes of probability information by the first and second decoder unit that are repeatedly executed, estimates the information bit list of the original lower two bits for each of the calculation processes; an error correction unit (corresponding to a first R/S decoder 19 and a second R/S decoder 21) which carries out an error checking process on the estimated information bit list by using an error correction code, and terminates the repeating process at the time when a judgment shows that the estimation precision exceeds a predetermined reference, as well as simultaneously carrying out an error-correction process on the estimated information bit list of the original lower two bits by using an error correction code; and a second estimating unit (corresponding to a third judging device 22) which hard-judges the other upper bits in the received signal so as to estimate an information bit list of the original upper bits.
A communication device in accordance with the next invention, which serves as a receiver using the turbo codes as error-correction codes, is provided with: a first decoder unit which extracts an information bit list of the two bits and a first redundant bit list from a received signal, and calculates the probability information of estimated information bits by using the results of the extraction and probability information that has been given as preliminary information (in some cases, not given); a second decoder unit which extracts the information bit list of the two bits and a second redundant bit list, and again calculates the probability information of estimated information bits by using the results of the extraction and the probability information from the first decoder unit to inform the first decoder unit of the results as the preliminary information; a first estimating unit which, based upon the results of the calculation processes of probability information by the first and second decoder unit that are repeatedly executed, estimates the information bit list of the original lower two bits for each of the calculation processes; an error correction unit which carries out an error checking process on the estimated information bit list by using an error correction code, and terminates the repeating process at the time when a judgment shows that the estimation precision exceeds a predetermined reference, as well as simultaneously carrying out an error-correction process on the estimated information bit list of the original lower two bits by using an error correction code; and a second estimating unit which hard-judges the other upper bits in the received signal so as to estimate an information bit list of the original upper bits.
In a communication device in accordance with the next invention, the error correction unit carries out an error checking process each time the information bit list of the lower two bits is estimated, and terminates the repeating process at the time when a judgment shows that xe2x80x9cno error existsxe2x80x9d in the estimated information bit list.
In a communication device in accordance with the next invention, the error correction unit carries out an error checking process each time the information bit list of the lower two bits is estimated, and terminates the repeating process at the time when a judgment shows that xe2x80x9cneither the information bit list estimated based upon the probability information from the first decoding unit nor the information bit list estimated based upon the probability information from the second decoding unit includes any errorxe2x80x9d in the estimated information bit list.
In a communication device in accordance with the next invention, the error correction unit carries out the repeating process for a predetermined number of times, and after the bit error rate has been reduced, an error-correction process is carried out on the estimated information bit list of the original lower two bits by using error correction codes.
A communication device in accordance with the next invention, which serves as a receiver using the turbo codes as error-correction codes, is provided with: a turbo encoder which carries out a turbo encoding process on lower two bits in transmission data to output an information bit list of the two bits, a first redundant bit list generated in a first convolutional encoder having the information bit list of the two bits as an input and a second redundant bit list generated in a second convolutional encoder to which the respective information bit lists that have been subjected to interleave processes are switched and input.
A communication method in accordance with the next invention, which uses turbo codes as error-correction codes, is provided with: a turbo encoding step of carrying out a turbo encoding process on lower two bits in transmission data to output an information bit list of the two bits, a first redundant bit list generated in a first convolutional encoder having the information bit list of the two bits as an input and a second redundant bit list generated in a second convolutional encoder to which the respective information bit lists that have been subjected to interleave processes are switched and input; a first decoding step of extracting the information bit list of the two bits and the first redundant bit list from a received signal, as well as calculating the probability information of estimated information bits by using the results of the extraction and probability information that has been given as preliminary information (in some cases, not given); a second decoding step of further extracting the information bit list of the two bits and the second redundant bit list, as well as again calculating the probability information of estimated information bits by using the results of the extraction and the probability information from the first decoding step to inform the first decoding step of the results as the preliminary information; a first estimating step of estimating the information bit list of the original lower two bits for each of the calculation processes, based upon the results of the calculation processes of probability information by the first and second decoding steps that are repeatedly executed; an error-correction step of carrying out an error checking process on the estimated information bit list by using an error correction code, and terminating the repeating process at the time when a judgment shows that the estimation precision exceeds a predetermined reference, as well as simultaneously carrying out an error-correction process on the estimated information bit list of the original lower two bits by using an error correction code; and a second estimating step of hard-judging the other upper bits in the received signal so as to estimate an information bit list of the original upper bits.
In the communication method in accordance with the next invention, the error correction step is designed to carry out an error checking process each time the information bit list of the lower two bits is estimated, and also to terminate the repeating process at the time when a judgment shows that xe2x80x9cno error existsxe2x80x9d in the estimated information bit list.
In the communication method in accordance with the next invention, the error correction step is designed to carry out an error checking process each time the information bit list of the lower two bits is estimated, and also to terminate the repeating process at the time when a judgment shows that neither the information bit list estimated based upon the probability information from the first decoding step nor the information bit list estimated based upon the probability information from the second decoding step includes xe2x80x9cany errorxe2x80x9d in the estimated information bit list.
In the communication method in accordance with the next invention, the error correction step is designed to carry out the repeating process for a predetermined number of times, and after the bit error rate has been reduced, an error-correction process is carried out on the estimated information bit list of the original lower two bits by using error correction codes.